Today a vast spectrum of different telecommunication systems has evolved for both wired and wireless telecommunication. Telecommunication systems have e.g. been standardized in connection with the so-called second generation (2G) and third generation (3G) mobile phone systems. Information about 3G-technology (e.g. W-CDMA or CDMA2000) and 2G-technology (e.g. GSM) etc. can e.g. be found in specifications from the 3rd Generation Partnership Project (3GPP), see e.g. the web-site at www.3gpp.org.
Further development has produced techniques for enabling even higher data transfer speeds. One such example is the ongoing development of the SAE/LTE (System Architecture Evolution/Long Term Evolution), which is the next step in terms of user-service experience, improving latency, capacity and throughput. For example, this includes the 3GPP work on the Evolution of the 3G Mobile Systems and hence the evolution of the Universal Terrestrial Radio Access Network (UTRAN).
As can be seen in FIG. 1, the evolved UTRAN comprises eNBs (eNode B) 1, providing the evolved UTRA User-plane (U-plane) and Control-plane (C-plane) protocol terminations towards the User Equipment (UE). The eNBs are interconnected with each other by means of a X2 interface 9. It is assumed that there always exist an X2 interface between the eNBs that need to communicate with each other, e.g. for support of handover of UEs in LTE ACTIVE. The eNBs are also connected by means of the S1 12 interface to the EPC (Evolved Packet Core). The S1 interface supports a many-to-many relation between aGWs (Access Gateways) and eNBs.
The eNB host various functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic Resource Allocation (scheduling), and so on as understood by the skilled person.
Mobility Management entity (MME) hosts various functions for distribution of paging messages to the eNBs.
User Plane Entity (UPE) hosts various functions for:                IP Header Compression and encryption of user data streams;        Termination of U-plane packets for paging reasons;        Switching of U-plane for support of UE mobility.        
Additional information can e.g. be found in the specification “3GPP TR 25.912 V7.1.0 (2006-09)” and in other specifications from the 3GPP associated therewith.
In this connection it has been agreed that Radio Resource Control (RRC) messages, exchanged between the eNode B and terminal (UE), should be ciphered and integrity protected. This requires that RRC keys are used in the eNode B and UE to perform the security functions. The RRC keys are generated in the Core Network (CN) and UE and are sent down from the CN to eNode B when the UE enters active state. The RRC keys are also sent between the eNode Bs during active mode intra-LTE mobility. The RRC is part of a sub layer of Layer 3 on the radio interface; it exists in the control plane only and provides information transfer service to the NAS (Non Access Stratum). RRC is responsible for controlling the configuration of radio interface Layers 1 and 2. The Non Access Stratum is a functional layer running and supporting traffic and signalling between the UE (User Equipment) and the CN (Core Network).
The ciphering and integrity protection algorithm requires a unique sequence number as input for each RRC message. The same sequence number and RRC key should never be used twice; however the same sequence number can be used as input to both the ciphering and the integrity protection.
Parts of the sequence number will be sent over the radio interface with every RRC message in order to key the sequence number synchronized in the sender and receiver, however in order to limit the number of bits sent over the radio interface it is possible to use a hyper frame number (HFN) (i.e. an overflow counter mechanism) which is not transferred over the radio but is maintained internally in the eNode B and terminal (UE). The HFN will also be used as input to the ciphering and integrity protection algorithm. The HFN will be a counter with enough number of bits so that the sequence number used as input to the ciphering and integrity protection algorithm will be unique within the life time of the RRC key.
The RRC key is generated during Network Attach or other core network procedure, by the Authentication and Key Agreement algorithm (AKA), which involves the (U)SIM card in the terminal and the HLR/HSS and other core network nodes.
This process is time consuming and it would be beneficial not to need to re-generate the RRC key after different mobility events such as handover and Idle to Active state transitions.
One state of the art solution used to be able to maintain RRC security during mobility events exists in the WCDMA/UMTS standard. This solution is based on;                a) Maintaining a START value in the UE/USIM which is used to initiate the HFN counter after an Idle to Active state transition. The START value is transferred to UTRAN during RRC connection setup. The HFN is always initiated to a value that is higher than the previously used SFN in order to avoid using the same HFN with the same RRC key.        b) During inter-RNC handovers the HFN is transferred to the target RNC, the HFN is also normally incremented by one or two steps during handover in order to avoid that the same HFN is re-used for the same RRC keys. This is due to that during the handover process the HFN could be incremented in the source RNC while the resources are being prepared in the target RNC.        
This solution is however fairly complex and requires additional security related signaling. One particular problem with the current solutions is that the HFN are used for multiple things, both as an overflow counter for the shorter sequence number used over the radio, but it is also incremented during mobility events such as handovers and idle to active state transitions.
For SAE/LTE which have a slightly different functional division between the core network and radio network (e.g. there are no RNCs) and for other standardised telecommunication networks having the same or similar abilities it is beneficial to utilize a different method.